Clay-Engine (merge clay-engine)

Initial work by Clément Foucault with contributions from Dalai Felinto
(mainly per-collection engine settings logic, and depsgraph iterator placeholder).

This makes Blender require OpenGL 3.3. Which means Intel graphic card
and OSX will break. Disable CLAY_ENGINE in CMake in those cases.

This is a prototype render engine intended to help the design of real
render engines. This is mainly an engine with enphasis in matcap and
ambient occlusion.

Implemented Features
--------------------

* Clay Render Engine, following the new API, to be used as reference for
future engines

* A more complete Matcap customization with more options

* Per-Collection render engine settings

* New Ground Truth AO - not enabled

Missing Features
----------------

* Finish object edit mode
  - Fix shaders to use new matrix
  - Fix artifacts when edge does off screen
  - Fix depth issue
  - Selection sillhouette
  - Mesh wires
  - Use mesh normals (for higher quality matcap)
  - Non-Mesh objects drawing
  - Widget drawing
  - Performance issues

* Finish mesh edit mode
  - Derived-Mesh-less edit mode API (mesh_rende.c)

* General edit mode
  - Per-collection edit mode settings

* General engines
  - Per-collection engine settings
    (they are their, but they still need to be flushed by depsgraph, and
    used by the drawing code)

CMakeLists.txt

@@ -242,6 +242,8 @@ endif()
 option(WITH_PLAYER        "Build Player" OFF)
 option(WITH_OPENCOLORIO   "Enable OpenColorIO color management" ${_init_OPENCOLORIO})
 
+option(WITH_CLAY_ENGINE    "Enable New Clay engine (Breaks Mac and Intel compatibility)" ON)
+
 # Compositor
 option(WITH_COMPOSITOR         "Enable the tile based nodal compositor" ON)